What do you get when you cross a tech blogger with someone intelligent? Why Daniel Rutter, who has run the numbers on the Gravia Lamp concept which won second place at the Greener Gadgets competition. Turns out the gravity-powered LED lamp pretty much could never work.
22.7 kilograms falling 1.22m in gravity of 9.8 metres per second squared gives you a grand total of 271.4 joules.
That, once again ignoring losses (which are likely to be considerable, seeing as there’s a ball-screw and an electrical generator in the Gravia), will by definition run a one-watt lamp for 271.4 seconds, or four and a half minutes.
If you downgrade the lamp to one tiny 0.1-watt LED night-light, you get three-quarters of an hour.
The maximum possible luminous efficacy for any kind of lamp that will ever exist – if every quantum of energy going into the thing is used to make visible photons that come out – is 683 lumens per watt. And that’s for a lamp that emits monochromatic 555-nanometre green light, not white (the world record for white LEDs in the lab so far is less than 150lm/W), but never mind that for now.
So if your tenth-watt lamp is just such a perfect device that can never actually exist, it will emit 68.3 lumens of light.
Look at all the shiny maths! Another critic pointed out that the Gravia would either need a 4,000-kilogram mass to drop 1.5 meters to power the lamp for four hours or to extend the design’s track for its 50-pound mass to 259 meters.
In fairness, the inventor has published a retraction and will give back the $1,000 Greener Gadgets prize. And I certainly didn’t realize there was anything wrong with the design. I got about as far as “Huh, I wonder why nobody ever used such a clever weighted system before?” and never gave it any more thought. I mean, if science can invent magical pipes that whistle when you invert them, surely lamps are just a step away?